I would strongly recommend against using Robot underground for reconing your Tumult. I've met Patrick before and he seems like a nice guy and everything. However, he's using the wrong coil design. XBL^2 design are sensitivity to using the right coil length and he's not even close. This totally screws up the BL curve (creating a huge BL peak in the center 5 mm or so of travel).
I'd definitely talk to Scott at
www.ficaraudio.com (or in his forum here). I believe that he still stocks that coil. If not, he could use the REXXX aluminum coil (but with a single soft spider).